REEMERGENCE OF EPIDEMIC VENEZUELAN EQUINE ENCEPHALOMYELITIS IN SOUTH-AMERICA

Background Venezuelan equine encephalomyelitis (VEE) virus has caused periodic epidemics among human beings and equines in Latin America fro m the 1920s to the early 1970s. The first major outbreak since 1973 oc curred in Venezuela and Colombia during 1995, and involved an estimate d 75 000 to 100 000 people. We report an and virological investigation of this epidemic. Methods Virus isolates were made in cell culture fr om human serum, human throat swabs, and brain tissue from aborted and stillborn human fetuses, as well as from horse brain tissue and pooled mosquito collections. Human sera were also tested for VEE-specific an tibodies. The serotypes of VEE isolates were identified by antigen ass ays, and viruses were characterised genetically by sequencing PCR prod ucts generated from the E3 and E2 genes. Phylogenetic analyses were do ne to determine evolutionary relations with respect to previous epidem ic/epizootic and enzootic VEE virus isolates. Mosquito collections wer e made to identify possible vectors, and clinical findings were determ ined by direct observation of patients visiting hospitals and clinics in affected regions, and by inspecting patient records. Equine vaccina tion and vector control were used in an attempt to halt the spread of the outbreak. Findings Most affected people had an acute, self-limited febrile illness of 3 to 4 days duration. However, convulsions were of ten seen in children, and abortions and fetal deaths occurred in pregn ant women infected with VEE virus. Antigenic characterisation of 12 vi rus isolates spanning the temporal and spatial range of the outbreak i ndicated that all are VEE serotype IC. Phylogenetic analysis revealed that all of the 1995 viruses were closely related to serotype IC virus es isolated during a large VEE outbreak that occurred in the same regi ons of Colombia and Venezuela from 1962-1964. A 1983 mosquito isolate from north central Venezuela was also closely related to the 1995 isol ates. Interpretation This outbreak was remarkably similar to one that occurred in same regions of Venezuela and Colombia during 1962-1964. S ymptoms of infected patients, estimated mortality rates, meteorologica l conditions preceding the epidemic, and seasonal patterns of transmis sion were all very similar to those reported in the previous outbreak. In addition, viruses isolated during 1995 were antigenically and gene tically nearly identifical to those obtained during 1962-1964. These f indings suggest that the epidemic resulted from the re-emergence of an epizootic serotype IC VEE virus. Identification of a similar virus is olate in mosquitoes in Venezuela in 1983, 10 years after epidemic/epiz ootic VEE activity ceased, raises the possibility of a serotype IC enz ootic transmission cycle in northern Venezuela.